Asteroidal assassins
People have been worrying about cosmic collisions for a long time, but the field only gained scientific legitimacy in the 1980s when the extinction of the dinosaurs was pinned on a single strike of a comet or asteroid. (Asteroids are rocky; comets are dirty snowballs. because they cause similar results when they hit Earth, and asteroids are more common, we'll refer to both bodies as "asteroids.")

Now we have the assertion, based on a computer simulation by Australian engineer Michael Paine, that during the last 10,000 years, Earth was hit about 350 times by asteroids as large as the rock that wasted 2,000 square kilometers of Siberian forest in 1908. According to the simulation, during the next 10,000 years, cosmic junk could kill 13 million people, and perhaps cause wars, famines and general-purpose chaos. Although the assertion was not published in a refereed journal, the alarming news was discussed at a national scientific meeting in February.

Before we get to the details, let's jump back in time with our we're-paranoid-for-a-reason collision chronology:

65 million years ago -- A 10-kilometer asteroid strikes north of the Yucatan Peninsula, causes a global firestorm, then a cold snap and finally a global warming that extinguishes the dinosaurs. Mammals move to center stage.
Some eventually make important discoveries like bingo, the Edsel, and the painful effects of asteroids.

3.3 million years ago -- An impact in Argentina precedes numerous extinctions and a global cooling trend (more on this later).

1490 -- About 10,000 people die in the Chinese city of Chi1ing-yang when an asteroid breaks overhead.

1908 -- An asteroid estimated at 50 meters across explodes above Tunguska, Siberia, blowing down trees across 2,000 square kilometers and killing a thousand reindeer, but apparently no people. Because the stony object exploded in the atmosphere, there's no crater.

1937 -- Asteroid Hermes -- about a kilometer in diameter -- misses Earth by 600,000 miles. Hermes, although smaller than the 'roid that snuffed the dinosaurs, could have been a true "category killer," able to cause epic devastation and kill millions.

1980 -- Spacewatch program starts at the University of Arizona, intent on cataloging asteroids. The goal is to get a statistical picture of orbiting rocks anywhere in the solar system.

1980 -- Physicist Luis Alvarez and his team blames the dinosaur extinction on the environmental havoc of a collision. The resulting firestorm and a global soot and dust cloud, they argue, cooled the planet enough to make the dinos long for a package vacation in Cancun. Many scientists, including renowned comet hunters, smirk in their beer about this ridiculous notion, which only gains acceptance after a 180-kilometer wide crater is discovered north of the Yucatan.

1994 -- Comet Shoemaker-Levy 9 breaks apart, then smashes into Jupiter under the watchful eye of dozens of telescopes. The resulting zone of chaos is estimated to be as large as the Earth and lends urgency to the search for asteroids and comets. "Shoemaker-Levy was a turning point," says Benny Peiser, an anthropologist at Liverpool John Moore's University at the 2000 meeting
of the American Association for the Advancement of Science in Washington, D.C. In the course of presenting results of the computer simulation on asteroid impacts, Peiser adds, "If it can happen in front of your nose -- practically in your backyard -- it can also happen on Earth."

1998 -- Astronomers announce that an asteroid may be on a collision course for Earth. The warning is quickly withdrawn after further observations.

1998 -- Peter Schultz, a professor of planetary geology at Brown University, associates greenish glass bodies found in Argentina with the extinction of 36 local animals (including one
we'd love to see, a carnivorous, flightless bird). The glass contains iridium, the same chemical that helped prove the impact theory of dinosaur extinction.
Still, correlation is not proof. "The climate change -- the sudden, dramatic cooling came immediately after the impact," Schultz says. "My gut says it's direct cause and effect, but we were careful to call this a coincidence."

2000 -- NASA's Near-Earth Asteroid Tracking System announces new data about large asteroids. "Until now, scientists thought the population of large, near-Earth asteroids was between 1,000 and 2,000, but we've downgraded that number significantly," said David Rabinowitz, now at Yale University. "We now believe there are between 500 and 1,000 near-Earth asteroids larger than one kilometer in diameter."

2000 -- Michael Paine, an Australian engineer, announces a new computer analysis of asteroid impacts indicating that asteroids may cause considerable chaos over a 100,000-year period. Most disturbing was a 5-kilometer asteroid, which exploded with a power of 23 million megatons, easily enough to wipe out the human population. Paine ignored this unsettling occurrence, since it was highly unlikely to occur
in any given 100,000-year span. Still, he estimated the annual risk of a fatal asteroid impact at one in 90, and concluded that an average of 120,000 people died per event.
A particular concern was tsunamis. When an asteroid hits the ocean, which covers about 70 percent of the planet, it can trigger a tsunami. According to the simulation, the average tsunami would kill 470,000 people.

The Why Files hates to base a story on a computer projection -- especially one done by a newcomer to the field of impact studies -- but when it comes to asteroidal impact, there's little else to go on. As we'll see, only about 3 percent of impacts leave a crater, and even when a crater does form, it is eventually buried by sediment, as happened to the Yucatan crater, or by the shifting of tectonic plates. On Earth, crater-counting can cause a false sense of security.

Have asteroids terminated civilizations? What to make of the terrestrial impact of cosmic impacts?